This application is based on Japanese Patent Application 2000-19062, filed on Jan. 27, 2000, and Japanese Patent Application 2000-258854, filed on Aug. 29, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a technology in which a laser beam is converged into a transparent material to change an optical property of the beam-converged portions of the material so as to form a mark within the transparent material.
2. Description of the Related Art
As a method for forming a mark within a transparent material, there have been known several processes disclosed in Japanese National Publication HEI 6-500275, JP-A HEI 7-136782 and JP Patent No. 2810151. Each of these disclosed processes is to converge a laser beam into a transparent material to form a crack within the material by virtue of a non-linear absorption effect. In this way, an opaque portion will result due to the formation of the crack, thereby forming a desired mark within the transparent material. On the other hand, in order to form a clearly visible mark in a transparent material using any one of the aforementioned processes, it is necessary to enlarge the size of a crack. However, once the crack is made large in its size, the strength of the transparent material will become low because the crack can probably extend to the surface of the transparent material, thus bringing about a damage (destruction) to the material.
In view of the above, the inventors of the present invention have tried an improved laser radiation process capable of performing a specific control of a laser beam, so that a crack is formed only in an internal position of a glass substrate even if it has a small thickness, as disclosed in JP-A HEI 11-13896.
Further, the inventors of the present invention have also developed another method which does not need to form a crack, but needs to change the refractive index of an object material so as to form a desired mark, as disclosed in JP-A HEI 11-267861. With the use of this method, since the crack is not formed, it is allowed to further reduce the possibility of causing a damage to an object material, as compared with the aforementioned method disclosed in JP-A HEI 11-13896.
However, in a process for carrying out the above method using a change in the refractive index, a mark will become extremely small in its size. As a result, a specific reading apparatus will become necessary to detect the presence of the small mark. Namely, although the aforementioned method can be suitably used to form a xe2x80x9chidden markxe2x80x9d, it is not suitable for general use in forming a common mark.
Accordingly, it is an object of the present invention to provide an improved marking method and an improved marking apparatus, capable of easily detecting the presence of a mark without using a specific reading apparatus, also capable of avoiding a damage as well as a strength deterioration of an object material. It is another object of the present invention to provide an indication apparatus in which marks are formed by using the use of the improved method and the improved apparatus.
According to one aspect of the present invention, there is provided a marking method comprising the steps of: preparing a marking object; converging, into the marking object, a laser beam having a wave length range so chosen that the laser beam can transmit through a material forming the marking object, and then effecting a multiple photon absorption; and moving a converging position of the laser beam, in a manner such that an area, whose refractive index is changed due to the multiple photon absorption, can form a diffraction pattern capable of diffracting a visible light.
According to another aspect of the present invention, there is provided a marking apparatus comprising: a stage for mounting a marking object; a light source for producing a laser beam having a wave length range so chosen that the laser beam can transmit through a material forming the marking object; an optical system for converging the laser beam emitted from the light source into the marking object, then effecting a multiple photon absorption; and a moving mechanism for moving the converging position in a manner such that characteristic-changed portions formed by virtue of the multiple photon absorption, will form, in the converging positions of the laser beam, a diffraction grating capable of diffracting a visible light.
The above method and the above apparatus involve converging, into the marking object, a laser beam having a wave length range so chosen that the laser beam can transmit through a material forming the marking object, and then effecting a multiple photon absorption. As a result, even if a laser beam has a relatively low photon energy, provided that such a laser beam is used and thus a light converging spot is formed within the marking object, it is still possible to effect a large change in an optical property (such as refractive index) of the marking object. Such change in an optical property is different from an occurrence of crack, so that it does not bring about any damage to the marking object or cause any deterioration in its strength. Further, since the above method ensures that an area with its refractive index changed can diffract a visible light, a mark formed within the marking object can be macroscopically recognized according to changes in brightness or darkness as well as changes in color. As a result, such a mark is easily visible.
According to a further aspect of the present invention, there is provided an optical element formed by a material capable of transmitting therethrough a visible light and containing a pattern consisting of different portions having different optical properties, wherein said pattern is capable of diffracting a visible light.
By virtue of diffraction of visible light, it is possible to visualize, according to changes in brightness or darkness as well as changes in color, a mark consisting of a pattern formed by portions of which different optical properties are changed. In practice, such a mark can be used as a mark identifying an apparatus. Further, it is also possible to manufacture an optical element having a colorful mark, as if it is an ornamental article.
According to a still further aspect of the present invention, there is provided a marking method comprising: a first step of irradiating a marking object by a pulse laser beam while at the same time changing NA of an objective lens and an energy per pulse, thereby forming a characteristic-changed area by virtue of movement of a light converging spot; a second step of obtaining a relationship among the length of the characteristic-changed area, an NA and an energy per pulse; a third step of determining the length of a characteristic-changed area to be formed; a fourth step of determining an NA and an energy per pulse, in accordance with the relationship obtained in the second step, also in accordance with the length of a characteristic-changed area to be formed; and a fifth step of converging a laser beam on the marking object to form a characteristic-changed area therein, by using the determined NA and the determined energy per pulse.
In this way, by properly selecting an NA and an energy per pulse, it is possible to form a characteristic-changed area having a desired length.